The present disclosure relates to aircraft guidance and control systems, in general, and in particular, to a command and control approach system that enables a manned or unmanned aircraft operating close to the ground or the deck of a moving ship to safely and precisely drop off or pick up an external cargo load that blocks the aircraft's direct, line-of-sight (LOS) sensors to the ground or ship.
In a low-speed, external cargo-hook operation of a helicopter, it is difficult to obtain an accurate above-ground-level (AGL) altitude because the cargo load typically interferes with the usual AGL altitude sensors of the vehicle. Such sensors, including Radar Altitude, Ultrasonic Altitude and Laser Altitude sensors, all provide relatively accurate AGL altitude, but need a direct, line-of-sight path to the ground to determine the distance between, e.g., a cargo hook or the wheels or skids of the helicopter and the ground.
This problem is more exacerbated when operating an unmanned helicopter or other lowspeed aerial vehicle, such as an unmanned aerial vehicle (UAV) with low speed or hovering capabilities, close to the ground, and either with or without an external cargo load. A number of problems can arise when carrying a cargo load with a manned or unmanned helicopter.
First, and most important, is that the system controlling the helicopter must know the precise location of the ground relative to the vehicle when the latter is operating very close to it. When carrying an external cargo load, conventional AGL altitude sensors do not work well because the load often obscures their view of the ground. This interference causes the conventional altitude sensors to indicate height above the load, rather than height above the ground. This renders conventional AGL altitude sensors relatively useless for any flight control system when operating close to the ground with an external load. Other altitude information sources, such as Barometric Altitude or GPS altitude, when coupled with a terrain data base, can be used to provide AGL altitude estimates, but these sources have errors that are sufficiently large that they cannot be relied upon for near-Earth operations. Additionally, it is difficult to locate AGL altitude sensors aboard an aircraft such that external loads will not interfere under all conditions.
A second problem is in providing a convenient and efficient command and control mechanism at a remote ground station for operating an unmanned helicopter relative to a load to be picked up from or dropped off onto a landing zone. The remote ground station operator must be able to position the helicopter precisely over the load to be picked up, and conversely, to be able to drop the load off at a precise location on the ground.
A third problem is in providing a redundant source of AGL altitude for an unmanned helicopter when operating close to the ground, even when it not carrying an external cargo load. Typically, a radar altimeter is used for low altitude operations, and it is of primary importance for a helicopter to have AGL altitude to effect precision operations and landings. If the radar altimeter is the only source of AGL altitude, the loss of this sensor becomes very critical with respect to the aircraft's flight operations. A radar altimeter failure during an approach, while hovering, or when landing, could result in severe damage to or loss of the vehicle. Thus, a redundant, independent and precise AGL altitude source would greatly reduce the probability of such an occurrence.
Another problem relates to helicopter operations carried out on or near a moving platform, such as the deck of a ship. Continuous, precise position and velocity updates of a moving ship are needed for feedback to the helicopter's control system to enable it to accurately maintain the position and velocity of the helicopter relative to a landing, pickup or drop-off zone on a moving ship.